Ionic liquids containing quaternary phosphonium cations and carboxylate anions, and their use as lubricant additives

ABSTRACT

The invention provides an ionic liquid composition having the following generic structural formula (1): 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , R 3 , and R 4  are each independently a hydrocarbon group containing at least 4 carbon atoms, and R 5  is a branched hydrocarbon group containing 4 to 8 carbon atoms atoms. The invention also provides a lubricant composition containing an ionic liquid dissolved in a base oil.

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to the fields of ionic liquids,and more particularly, to their application as additives in lubricatingoils, such as engine and motor oils.

BACKGROUND OF THE INVENTION

Friction and wear account for ˜6% of U.S. GNP. Particularly, in aninternal combustion engine, 10-15% energy is lost to parasitic friction.In 2009, it was estimated that 208,000 million liters of fuel wasconsumed to overcome the friction in passenger cars worldwide. One wayto boost the engine efficiency is to use a lower viscosity lubricatingoil, which also challenges the wear protection of engine bearingcomponents. Therefore, developing a new class of more effectiveanti-wear (AW) lubricant additives is of great interest from bothfundamental and practical perspectives in energy savings.

Ionic liquids have been explored as lubricant additives for at least thelast decade. Ionic liquids are composed of cations and anions. The fourcommonly used cations are phosphonium, ammonium, pyridinium, andimidazolium; and the forms of anions vary. Ionic liquid lubrication hasmainly been explored in neat form or as base stocks. Many ionic liquidspossess high thermal stability that allows them to be potentiallyapplied for high-temperature lubrication where traditional hydrocarbonlubricants are unstable.

However, ionic liquids tend to have low solubility in common base oils,which is a significant obstacle to their use since the lowconcentrations used and/or incomplete miscibility results in substandardor inconsistent wear and friction control. Thus, there is a need forimproving the solubility of ionic liquids in various lubricating oils.Moreover, there is a need for new ionic liquid compositions havingimproved anti-wear and friction reduction properties.

SUMMARY OF THE INVENTION

In one aspect, the invention is directed to an ionic liquid compositionhaving the following generic structural formula (1):

wherein R¹, R², R³, and R⁴ are each independently a hydrocarbon groupcontaining at least 4 carbon atoms, and R⁵ is a branched hydrocarbongroup containing 4 to 8 carbon atoms atoms.

In another aspect, the invention is directed to a lubricant compositioncomprising (i) an ionic liquid having the following generic structuralformula (1):

wherein R¹, R², R³, and R⁴ are each independently a hydrocarbon groupcontaining at least 4 carbon atoms, and R⁵ is a branched hydrocarbongroup containing 4 to 8 carbon atoms; and (ii) a base oil; wherein theionic liquid is dissolved in the base oil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B: Molecular structures of trihexyltetradecylphosphonium[C₇H₁₅COO] (A) branched and (B) straight chained version.

FIGS. 2A, 2B, 2C: Three different anion alkyls. (A) branched [C₉H₁₉COO];(B) straight [C₉H₁₉COO]; and (C) straight [C₁₇H₃₅COO].

FIG. 3: Branched C7 phosphonium-carboxylate IL ([P₆₆₆₁₄][C₇H₁₅COO]b)outperforms a commercial amine-phosphate AW additive.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F: TEM (A,B) and EDS (C) examination on thecross-section of the worn surface lubricated by PAO+[P₆₆₆₁₄][C₇H₁₅COO]b.(B) corresponds to the dotted box in (A). The EDS cross-sectionalelemental maps show concentrations of iron (D), oxygen (E) andphosphorus (F).

FIGS. 5A, 5B, 5C, 5D, 5E. XPS detailed spectra of (A) Fe2p, (B) O1s, and(C) C1s, and (D) P2p core levels on the worn surface lubricated byPAO+[P₆₆₆₁₄][C₇H₁₅COO]b; (E) composition-depth profile.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “about” generally indicates within ±0.5%, 1%,2%, 5%, or up to ±10% of the indicated value. For example, the term“about 100° C.” generally indicates, in its broadest sense, 100° C.±2%,which indicates 98-112° C. The term “about” may alternatively indicate avariation or average in a physical characteristic of a group.

In one aspect, the invention is directed to an ionic liquid useful as alubricant additive or lubricant itself. As understood in the art, theterm “ionic liquid compound” or “ionic liquid” is an ionic compound thatis, itself, a liquid, i.e., without being dissolved in or solvated witha solvent. The ionic liquid is typically a liquid at room temperature(e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. However, in someembodiments, the ionic liquid may become a liquid at a temperature above30° C. Thus, in some embodiments, the ionic liquid may have a meltingpoint of up to or less than 100, 90, 80, 70, 60, 50, 40, or 30° C. Inother embodiments, the ionic liquid is a liquid at or below 10, 5, 0,−10, −20, −30, or −40° C.

The density of the ionic liquid is typically in the range of 0.6-1.6g/mL at an operating temperature of interest, and particularly at atemperature within 20-40° C. The viscosity of the ionic liquid istypically no more than 50,000 centipoise (50,000 cP) at an operatingtemperature of interest, and particularly at a temperature within 20-40°C. In different embodiments, the viscosity of the ionic liquid may beabout, up to, less than, at least, or above, for example, 50, 100, 200,300, 400, 500, 600, 700, 800, 900, 1000, 2000, 5000, 10,000, 15,000,20,000, or 25,000 cP, or a viscosity within a range bounded by any twoof these values.

The ionic liquid composition of the invention contains a quaternaryphosphonium cation and a carboxylate containing anion. In particularembodiments, the ionic liquid compositions are conveniently described bythe following generic structural formula (1):

Quaternary Phosphonium Cation

In Formula (1) above, R¹, R², R³, and R⁴ are each independently ahydrocarbon group containing at least 4 carbon atoms. The term“hydrocarbon group” or “hydrocarbon linker” as used herein for R¹, R²,R³, and R⁴, designates, in a first embodiment, groups or linkerscomposed solely of carbon and hydrogen. In different embodiments, one ormore of the hydrocarbon groups or linkers can contain precisely, or aminimum of (i.e., at least), or a maximum of (i.e., up to), for example,four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen,fourteen, fifteen, sixteen, seventeen, eighteen, nineteen, or twentycarbon atoms, or a number of carbon atoms within a particular rangebounded by any two of the foregoing carbon numbers. Hydrocarbon groupsor linkers in different compounds described herein, or in differentparts or positions of a compound, may possess the same or differentnumber (or preferred range thereof) of carbon atoms in order toindependently adjust or optimize the activity or other characteristicsof the compound, such as its level of hydrophobicity or solubility levelin a hydrophobic medium, or its wear-enhancing or friction-reducingability.

The hydrocarbon groups or linkers in R¹, R², R³, and/or R⁴ can be, forexample, saturated and straight-chained, i.e., straight-chained alkylgroups or alkylene linkers. Some examples of straight-chained alkylgroups (or alkylene linkers) include n-propyl, n-butyl, n-pentyl,n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl,n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl,n-octadecyl, n-nonadecyl, and n-eicosyl groups (or their respectivelinker analogs).

The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/orR⁴ can alternatively be saturated and branched, i.e., branched alkylgroups or alkylene linkers. Some examples of branched alkyl groupsinclude isopropyl(2-propyl), isobutyl(2-methylprop-1-yl),sec-butyl(2-butyl), t-butyl, 2-pentyl, 3-pentyl, 2-methylbut-1-yl,isopentyl(3-methylbut-1-yl), 1,2-dimethylprop-1-yl,1,1-dimethylprop-1-yl, neopentyl(2,2-dimethylprop-1-yl), 2-hexyl,3-hexyl, 2-methylpent-1-yl, 3-methylpent-1-yl,isohexyl(4-methylpent-1-yl), 1,1-dimethylbut-1-yl, 1,2-dimethylbut-1-yl,2,2-dimethylbut-1-yl, 2,3-dimethylbut-1-yl, 3,3-dimethylbut-1-yl,1,1,2-trimethylprop-1-yl, 1,2,2-trimethylprop-1-yl, 2-ethylhexyl,isoheptyl, isooctyl, isononyl, and isodecyl, wherein the “1-yl” suffixrepresents the point of attachment of the group. Some examples ofbranched alkylene linkers are those derived by removal of a hydrogenatom from one of the foregoing exemplary branched alkyl groups, e.g.,isopropylene (—CH(CH₃)CH₂—).

The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/orR⁴ can alternatively be saturated and cyclic, i.e., cycloalkyl groups orcycloalkylene linkers. Some examples of cycloalkyl groups includecyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, andcyclooctyl groups. The cycloalkyl group can also be a polycyclic (e.g.,bicyclic) group by either possessing a bond between two ring groups(e.g., dicyclohexyl) or a shared (i.e., fused) side, e.g., decalin andnorbornane. Some examples of cycloalkylene linkers are those derived byremoval of a hydrogen atom from one of the foregoing exemplarycycloalkyl groups.

The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/orR⁴ can alternatively be saturated and cyclic, i.e., cycloalkyl groups orcycloalkylene linkers. Some examples of cycloalkyl groups includecyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, andcyclooctyl groups. The cycloalkyl group can also be a polycyclic (e.g.,bicyclic) group by either possessing a bond between two ring groups(e.g., dicyclohexyl) or a shared (i.e., fused) side, e.g., decalin andnorbornane. Some examples of cycloalkylene linkers are those derived byremoval of a hydrogen atom from one of the foregoing exemplarycycloalkyl groups.

The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/orR⁴ can alternatively be unsaturated and straight-chained, i.e.,straight-chained olefinic or alkenyl groups or linkers. The unsaturationoccurs by the presence of one or more carbon-carbon double bonds and/orone or more carbon-carbon triple bonds. Some examples ofstraight-chained olefinic groups include, 2-propen-1-yl(allyl),3-buten-1-yl(CH₂═CH—CH₂—CH₂—), 2-buten-1-yl(CH₂—CH═CH—CH₂—), butadienyl(e.g., 1,3-butadien-1-yl), 4-penten-1-yl, 3-penten-1-yl, 2-penten-1-yl,2,4-pentadien-1-yl, 5-hexen-1-yl, 4-hexen-1-yl, 3-hexen-1-yl,3,5-hexadien-1-yl, 1,3,5-hexatrien-1-yl, 4-hepten-1-yl, 5-hepten-1-yl,6-hepten-1-yl, 4-octen-1-yl, 5-octen-1-yl, 6-octen-1-yl, 7-octen-1-yl,2,6-octadien-1-yl, 8-decenyl, 9-decenyl, or 4,8-decadien-1-yl, ethynyl,propargyl(2-propynyl), and the numerous C₇, C₈, C₉, C₁₀, C₁₁, C₁₂, C₁₃,C₁₄, C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, and C₂₀ unsaturated and straight-chainedhydrocarbon groups. Some examples of straight-chained olefinic linkersare those derived by removal of a hydrogen atom from one of theforegoing exemplary straight-chained olefinic groups.

The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/orR⁴ can alternatively be unsaturated and branched, i.e., branchedolefinic or alkenyl groups or linkers. Some examples of branchedolefinic groups include 1-buten-2-yl(CH₂═C.—CH₂—CH₃),1-buten-3-yl(CH₂═CH—CH.—CH₃), 1-propen-2-methyl-3-yl(CH₂═C(CH₃)—CH₂.),1-penten-4-yl, 1-penten-3-yl, 1-penten-2-yl, 2-penten-2-yl,2-penten-3-yl, 2-penten-4-yl, 1,4-pentadien-3-yl, 2,4-pentadien-3-yl,3-methyl-2-buten-1-yl, 2,3-dimethyl-2-buten-1-yl,4-methyl-2-penten-1-yl, 2-hexen-5-yl, and the numerous C₇, C₈, C₉, C₁₀,C₁₁, C₁₂, C₁₃, C₁₄, C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, and C₂₀ unsaturated andbranched hydrocarbon groups. Some examples of branched olefinic linkersare those derived by removal of a hydrogen atom from one of theforegoing exemplary branched olefinic groups.

The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/orR⁴ can alternatively be unsaturated and cyclic (i.e., cycloalkenylgroups or cycloalkenylene linkers). The unsaturated and cyclic group canbe aromatic or aliphatic. Some examples of unsaturated and cyclichydrocarbon groups include cyclopropenyl, cyclobutenyl, cyclopentenyl,cyclopentadienyl, cyclohexenyl, cyclohexadienyl, phenyl, benzyl,cycloheptenyl, cycloheptadienyl, cyclooctenyl, cyclooctadienyl, andcyclooctatetraenyl groups. The unsaturated cyclic hydrocarbon group canalso be a polycyclic group (such as a bicyclic or tricyclic polyaromaticgroup) by either possessing a bond between two of the ring groups (e.g.,biphenyl) or a shared (i.e., fused) side, as in naphthalene, anthracene,phenanthrene, phenalene, or indene fused ring systems. Some examples ofunsaturated cycloalkenylene linkers are those derived by removal of ahydrogen atom from one of the foregoing exemplary cycloalkenyl groups(e.g., phenylene and biphenylene).

One or more of the hydrocarbon groups or linkers as used herein for R¹,R², R³, and/or R⁴ may (i.e., optionally) be substituted with (i.e.,include) one or more heteroatoms, which are non-carbon non-hydrogenatoms. Some examples of heteroatoms include oxygen (O), nitrogen (N),sulfur (S), and halogen (halide) atoms, wherein some examples of halogenatoms include fluorine, chlorine, bromine, and iodine. In someembodiments, the heteroatom atom inserts between at least two carbonatoms (as in —C—O—C— ether, —C—N(R)—C— tertiary amine, or —C(═NR)C—imine) or between at least one carbon atom and at least one hydrogenatom (as in —C—OH, —C—SH, —C—NH₂, —C—NH—C—, or —C(═NH)C—), wherein theshown carbon atom in each case can be considered part of a hydrocarbongroup described above. In other embodiments, the heteroatom replaces oneor more hydrogen atoms and/or one or more carbon atoms in thehydrocarbon group, as in halogen-substituted groups (e.g., as in —CH₂F,—CHF₂, and —CF₃) and carbonyl-substituted groups, such as ketone andaldehyde groups. In the case of nitrogen or sulfur substitution, thenitrogen or sulfur atom may be bonded to a sufficient number of groupsto make it positively charged, as in an ammonium group (e.g., —NR′₃ ⁺)or sulfonium group (e.g., —SR′₂ ⁺), in which case the positively chargedmoiety is necessarily associated with a counteranion, wherein R′independently represents hydrogen atom or any of the hydrocarbon groupsdescribed above. Likewise, a heteroatom may bear a negative charge, asin a deprotonated alkoxide or thio group, in which case the negativelycharged moiety is necessarily associated with a countercation.

When two or more same or different heteroatoms are bound to each otheror located on the same carbon atom, the resulting group containing theheteroatoms is herein referred to as a “heteroatom-containing group”.Thus, substitution with one or more heteroatoms also includesheteroatom-containing groups, unless otherwise specified. Some examplesof heteroatom-containing groups and linkers include carboxy (—C(O)OR′ or—OC(O)R′), carboxamide (—C(O)NR′₂, —C(O)NR′—, or —N(R′)C(O)—), urea(—NR′—C(O)—NR′₂ or —NR′—C(O)—NR′—), carbamate (—NR′—C(O)—OR′,—OC(O)—NR′₂, or —NR′—C(O)—O—), nitro (NO₂), nitrile (CN), sulfonyl(—S(O)₂R′ or —S(O)₂—), sulfinyl (i.e., sulfoxide, —S(O)R′ or —S(O)—),disulfide (—C—S—S—C—), sulfonate (—S(O)₂R′), and amine oxide (astypically found in a nitrogen-containing ring), wherein R′ canindependently represent, for example, hydrogen atom or any of thehydrocarbon groups described above. For example, —C(O)OR′ includescarboxylic acid (—C(O)OH) and carboxylic ester (—C(O)OR), wherein R canbe any of the hydrocarbon groups described above. Theheteroatom-containing group may also either insert between carbon atomsor between a carbon atom and hydrogen atom, if applicable, or replaceone or more hydrogen and/or carbon atoms.

In some embodiments, the hydrocarbon group or linker as used herein forR¹, R², R³, and/or R⁴ is substituted with one or more halogen atoms toresult in a partially halogenated or perhalogenated hydrocarbon group.Some examples of partially halogenated hydrocarbon groups include—CHX′₂, —CH₂X′, —CH₂CX′₃, —CH(CX′₃)₂, or a monohalo-, dihalo-, trihalo-,or tetrahalo-substituted phenyl group, wherein X′ represents any of F,Cl, Br, or I, and more commonly, F or Cl. Some examples ofperhalogenated hydrocarbon groups include —CX′₃, —CX′₂CX′₃,—CX′₂CX′₂CX′₃, —CX′(CX′₃)₂, or a perhalophenyl group —C₆X′₅.

In particular embodiments, the hydrocarbon group (R¹, R², R³, and/or R⁴)is, or includes, a cyclic or polycyclic (i.e., bicyclic, tricyclic, orhigher cyclic) saturated or unsaturated (e.g., aliphatic or aromatic)hydrocarbon group that includes at least one ring heteroatom, such asone, two, three, four, or higher number of ring heteroatoms. Suchheteroatom-substituted cyclic hydrocarbon groups are referred to hereinas “heterocyclic groups”. As used herein, a “ring heteroatom” is an atomother than carbon and hydrogen (typically, selected from nitrogen,oxygen, and sulfur) that is inserted into or replaces a ring carbon atomin a hydrocarbon ring structure. In some embodiments, the heterocyclicgroup is saturated, while in other embodiments, the heterocyclic groupis unsaturated, i.e., aliphatic or aromatic heterocyclic groups, whereinthe aromatic heterocyclic group is also referred to herein as a“heteroaromatic ring”, or a “heteroaromatic fused-ring system” in thecase of at least two fused rings, at least one of which contains atleast one ring heteroatom.

Some examples of saturated heterocyclic groups containing at least oneoxygen atom include oxetane, tetrahydrofuran, tetrahydropyran,1,4-dioxane, 1,3-dioxane, and 1,3-dioxepane rings. Some examples ofsaturated heterocyclic groups containing at least one nitrogen atominclude pyrrolidine, piperidine, piperazine, imidazolidine, azepane, anddecahydroquinoline rings. Some examples of saturated heterocyclic groupscontaining at least one sulfur atom include tetrahydrothiophene,tetrahydrothiopyran, 1,4-dithiane, 1,3-dithiane, and 1,3-dithiolanerings. Some examples of saturated heterocyclic groups containing atleast one oxygen atom and at least one nitrogen atom include morpholineand oxazolidine rings. An example of a saturated heterocyclic groupcontaining at least one oxygen atom and at least one sulfur atomincludes 1,4-thioxane. Some examples of saturated heterocyclic groupscontaining at least one nitrogen atom and at least one sulfur atominclude thiazolidine and thiamorpholine rings.

Some examples of unsaturated heterocyclic groups containing at least oneoxygen atom include furan, pyran, 1,4-dioxin, benzofuran, dibenzofuran,and dibenzodioxin rings. Some examples of unsaturated heterocyclicgroups containing at least one nitrogen atom include pyrrole, imidazole,pyrazole, pyridine, pyrazine, pyrimidine, 1,3,5-triazine, azepine,diazepine, indole, purine, benzimidazole, indazole, 2,2′-bipyridine,quinoline, isoquinoline, phenanthroline, 1,4,5,6-tetrahydropyrimidine,1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, quinoxaline,quinazoline, pyridazine, cinnoline, 5,6,7,8-tetrahydroquinoxaline,1,8-naphthyridine, and 4-azabenzimidazole rings. Some examples ofunsaturated heterocyclic groups containing at least one sulfur atominclude thiophene, thianaphthene, and benzothiophene rings. Someexamples of unsaturated heterocyclic groups containing at least oneoxygen atom and at least one nitrogen atom include oxazole, isoxazole,benzoxazole, benzisoxazole, oxazoline, 1,2,5-oxadiazole (furazan), and1,3,4-oxadiazole rings. Some examples of unsaturated heterocyclic groupscontaining at least one nitrogen atom and at least one sulfur atominclude thiazole, isothiazole, benzothiazole, benzoisothiazole,thiazoline, and 1,3,4-thiadiazole rings.

The positive (+) charge shown in Formula (1) resides on the phosphorus(P) atom shown in Formula 1. However, one or more additional positivecharges may exist elsewhere in the quaternary phosphonium cation, whichwould add to the overall positive charge of the quaternary phosphoniumcation. The quaternary phosphonium cation can be, for example, any ofthe phosphonium moieties disclosed in U.S. Pat. No. 3,654,342 and U.S.Pat. No. 3,459,795.

In one set of embodiments, the quaternary phosphonium cation istrihexyltetradecylphosphonium [P₆₆₆₁₄], which has the followingstructural formula (Formula (2)):

In another set of embodiments, the quaternary phosphonium cation istetraoctylphosphonium [P₈₈₈₈], which has the following structuralformula (Formula (3)):

In still another set of embodiments, the quaternary phosphonium cationis tributyltetradecylphosphonium [P₄₄₄₁₄], which has the followingstructural formula (Formula (4)):

In a further set of embodiments, the quaternary phosphonium cation istributyltetraoctylphosphonium [P₄₄₄₈], which has the followingstructural formula (Formula (5)):

Carboxylate Anion

In Formula (1) above, R⁵ is a branched hydrocarbon group having 4 to 8carbon atoms. The hydrocarbon group can be a saturated or unsaturatedbranched hydrocarbon group. Examples of saturated or unsaturatedbranched hydrocarbon groups having 4 to 8 carbon atoms include thosehydrocarbons described above for R¹, R², R³, and R⁴ having 4 to 8 carbonatoms. The branch can occur on any of the carbon atoms in thehydrocarbon group. For example, the branch can occur on the first,second, third, fourth, fifth, sixth or seventh carbon atom of R⁵.

In one set of embodiments, R⁵ is branched in an octanoate (C₇H₁₅COO).The branch can occur anywhere in the hydrocarbon moiety. For example,the branch can occur on the first, second, third, fourth, fifth, orsixth carbon atom of R⁵.

In one embodiment, branched C₇H₁₅COO is a 2-ethylhexanonate, asillustrated in Formula (6):

In Formula (6) above, the letter “b” in [C₇H₁₅COO]b denotes thatC₇H₁₅COO is in a branched configuration. The branch in Formula (6)occurs on the carbon in the second position of the chain (first positionof R⁵).

Ionic Liquid Composition

The ionic liquid composition of the invention includes any of the abovecationic phosphonium species (herein identified as L⁺) and any of theabove anionic species X⁻, in accordance with Formula (1). The ionicliquid composition can be conveniently expressed by the formula L⁺X⁻,wherein L⁺ is a cationic component of the ionic liquid and X⁻ is ananionic component of the ionic liquid. The formula (L⁺)(X⁻) is meant toencompass a cationic component (L⁺) having any valency of positivecharge, and an anionic component (X⁻) having any valency of negativecharge, provided that the charge contributions from the cationic portionand anionic portion are counterbalanced in order for charge neutralityto be preserved in the ionic liquid molecule. More specifically, theformula (L⁺)(X⁻) is meant to encompass the more generic formula(L^(+a))_(y)(X^(−b))_(x), wherein the variables a and b are,independently, non-zero integers, and the subscript variables x and yare, independently, non-zero integers, such that a.y=b.x (wherein theperiod placed between variables indicates multiplication of thevariables). The foregoing generic formula encompasses numerous possiblesub-formulas, such as, for example, (L⁺)(X⁻), (L⁺²)(X⁻)₂, (L⁺)₂(X⁻²),(L⁺²)₂(X⁻²)₂, (L⁺³)(X⁻)₃, (L⁺)₃(X⁻³), (L⁺³)₂(L⁻²)₃, and (L⁺²)₃(X⁻³)₂.

The ionic liquid compositions described above can be synthesized bymethodologies well known in the art. The methodologies typically involvesalt-forming exchange between cationic- and anionic-containing precursorcompounds. For example, a phosphonium halide compound of the formula[PR¹R²R³R⁴]⁺[X′]⁻ (where the halide X′ is typically chloride, bromide,or iodide) can be reacted with the acid or salt form of any of thecarboxylate-containing anions described above to form an ionic liquidaccording to Formula (1) above, with concomitant liberation of thecorresponding hydrogen halide or halide salt. Such methods aredescribed, for example, in J. Qu, et al., Applied Materials andInterfaces, 4, pp. 997-1002, 2012, which is herein incorporated byreference in its entirety.

The ionic liquid compositions described above possess completesolubility in a base oil when included in the base oil in amounts of atleast 0.1, 0.5, 1, 2, 5, 10, or 50 wt % or within a concentrationbounded by any two of these concentrations.

Lubricant

In another aspect, the invention is directed to a lubricant compositionthat includes one or more of the ionic liquid compositions describedabove dissolved in a base oil. The term “dissolved”, as used herein,indicates complete dissolution of the ionic liquid in the base oil,i.e., the ionic liquid is completely miscible in the base oil. Indifferent embodiments, the ionic liquid is dissolved in the base oil inan amount of at least 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35,40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90 wt % (i.e., weight ofionic liquid by weight of the total of ionic liquid and base oil) ordissolved in the base oil within a range bounded by any two of theforegoing values. Generally, the ionic liquid in the lubricantcomposition is one, two, or more selected from any of the ionic liquidsherein described, in the absence of other ionic liquids that do notpossess the features of the instantly described ionic liquids, such as aquaternary phosphonium cation component having at least four hydrocarbongroups or a carboxylate anion containing a hydrocarbon group having 4 to8 carbon atoms. In some embodiments, the lubricant composition havingany of the above concentrations of ionic liquids is used directly as alubricant without diluting in additional oil or organic solvent. Inother embodiments, the lubricant composition having any of the aboveconcentrations of ionic liquid is diluted before use. Thus, any of theabove-described lubricant compositions having any of the aboveconcentrations of ionic liquid (particularly those of higherconcentration, e.g., at least 10, 20, 30, 40, or 50 wt %) may be storedas a commodity, and optionally diluted, prior to use.

The base oil can be any of the polar or non-polar base oils known in theart useful as mechanical lubricating oils. As well known in the art, themechanical lubricating oil can be further classified as, for example, anengine (motor) lubricating oil, industrial lubricating oil, or metalworking fluid. The classification, uses, and properties of such oils arewell known in the art, as provided, for example, by U.S. Pat. No.8,268,760, the contents of which are herein incorporated by reference intheir entirety. In particular, the base oil may belong to any of thewell established five categories of hydrocarbon oils (i.e., Groups I,II, III, IV, or V) classified according to the extent of saturates,sulfur, and viscosity index. The base oil can have any of the typicalboiling points, e.g., at least 100, 120, 150, 180, or 200° C. and up to250, 300, 350, 400, 450, or 500° C. In some embodiments, the base oil isa synthetic oil, such as any of the Groups I-V, and may or may notinclude polyalphaolefins (PAO). Some other synthetic oils includehydrogenated polyolefins, esters, fluorocarbons, and silicones. In otherembodiments, the base oil may be natural, such as a mineral oil,vegetable oil, or animal oil. In yet other embodiments, the base oil mayhave a substantially high enough viscosity to qualify it as a grease,wherein the grease typically lowers in viscosity during use by virtue ofheat generated during use.

The lubricant composition may also include any one or more lubricantadditives well known in the art. The term “additive”, as used herein, isunderstood to be a compound or material, or mixture of compounds ormaterials, that provides an adjunct or auxiliary effect at lowconcentrations, typically up to or less than 1, 2, 5, 7, or 10 wt % byweight of the lubricant composition. The additive can be, for example,an anti-wear additive (typically metal-containing), extreme pressureadditive, metal chelator, ultraviolet stabilizer, radical scavenger,anti-oxidant, corrosion inhibitor, friction modifier, detergent,surfactant, anti-foaming agent, viscosity modifier, or anti-foamingagent, or combination thereof, all of which are well known in the art,as further described in U.S. Pat. Nos. 8,455,407 and 8,268,760, both ofwhich are herein incorporated by reference in their entirety.

In particular embodiments, the lubricating composition described aboveincludes a non-ionic liquid (non-IL) anti-wear additive, such as ametal-containing dithiophosphate, sulfur-containing fatty acid or esterthereof, dialkyl sulfide, dithiocarbamate, polysulfide, or boric acidester. In further embodiments, the additive is a metal-containingdialkyldithiophosphate or dialkyldithiocarbamate, wherein the metal istypically zinc or molybdenum, as in zinc dialkyldithiophosphate (ZDDP)or molybdenum dialkyldithiocarbamate (MoDTC), and the alkyl groupstypically include between 3 and 12 carbon atoms and can be linear orbranched. The anti-wear additive can be included in the lubricatingcomposition in any suitable amount typically used in the art, such asbetween 1 and 15 wt %. In some embodiments, the anti-wear additive isadvantageously used in an amount less than typically used in the art,e.g., in an amount of less than 1 wt %, or up to or less than 0.5 or 0.1wt %, by virtue of the improved properties provided by the instantlydescribed ionic liquids or by a synergistic interaction between theinstantly described ionic liquids and the non-IL anti-wear additive.

In one embodiment, the ionic liquid or the lubricating composition isnot dissolved, admixed with, or otherwise in contact with a non-ionicliquid organic solvent (i.e., “solvent”). In other embodiments, theionic liquid is dissolved in, or admixed with, or in contact with one ormore organic solvents, either in the absence or presence of a base oil.If the ionic liquid is dissolved in a base oil, then the organic solventshould be completely soluble in the base oil. The organic solvent canbe, for example, protic or non-protic and either polar or non-polar.Some examples of protic organic solvents include the alcohols,particularly those more hydrophobic than methanol or ethanol, such asn-propanol, isopropanol, n-butanol, sec-butanol, isobutanol, t-butanol,n-pentanol, isopentanol, 3-pentanol, neopentyl alcohol, n-hexanol,2-hexanol, 3-hexanol, 3-methyl-1-pentanol, 3,3-dimethyl-1-butanol,isohexanol, and cyclohexanol. Some examples of polar aprotic solventsinclude ether (e.g., diethyl ether, 1,2-dimethoxyethane,1,2-diethoxyethane, 1,3-dioxolane, and tetrahydrofuran), ester (e.g.,1,4-butyrolactone, ethylacetate, methylpropionate, and ethylpropionate),nitrile (e.g., acetonitrile, propionitrile, and butyronitrile),sulfoxide (e g, dimethyl sulfoxide, ethyl methyl sulfoxide, diethylsulfoxide, methyl propyl sulfoxide, and ethyl propyl sulfoxide), andamide solvents (e.g., N,N-dimethylformamide, N,N-diethylformamide,acetamide, and dimethylacetamide). Some examples of non-polar solventsinclude the liquid hydrocarbons, such as the pentanes, hexanes,heptanes, octanes, pentenes, hexenes, heptenes, octenes, benzene,toluenes, and xylenes.

In another aspect, the invention is directed to methods for using theabove-described ionic liquids, either autonomously (i.e., in the absenceof a base oil) or within a lubricant composition, for reducing wearand/or reducing friction in a mechanical device for which lubricity isbeneficial. The mechanical device may be, for example, a bearing (e.g.,a slide bearing, ball bearing, rolling element bearing, or jewelbearing), piston, turbine fan, rotary blade, compressor blade, gear,axle, engine part (e.g., engine valve, piston, cylinder, ortransmission), hydraulic system, or metal cutting tool or machine. Theparts being lubricated are typically constructed of a metal or metalalloy, which may be or include, for example, steel, iron, aluminum,nickel, titanium, or magnesium, or a composite or alloy thereof. If usedautonomously, the ionic liquid is not included in a base oil, but may becombined with any one or more of the additives described above if theionic liquid and additive are miscible with each other. The ionic liquidor lubricant composition described above can be applied to a mechanicalcomponent by any means known in the art. For example, the component maybe immersed in the ionic liquid compound, or a coating (film) of theionic liquid compound may be applied to the component by, e.g., dipping,spraying, painting, or spin-coating.

In some embodiments, a single ionic liquid compound according to Formula(1) is used, while in other embodiments, a combination of two or moreionic liquid compounds according to Formula (1) is used. In a firstincarnation, the combination of ionic liquid compounds corresponds tothe presence of two or more cationic species of any of those describedabove in the presence of a single anionic species of any of thosedescribed above. In a second incarnation, the combination of ionicliquid compounds corresponds to the presence of a single cationicspecies in the presence of two or more anionic species. In a thirdincarnation, the combination of ionic liquid compounds corresponds tothe presence of two or more cationic species of any of those describedabove in the presence of two or more anionic species of any of thosedescribed above.

The ionic liquids described above reduce wear and/or friction. In someembodiments, the ionic liquid or lubricating composition in which it isincorporated provides a coefficient of friction (i.e., frictioncoefficient) of up to or less than, for example, 0.5, 0.4, 0.3, 0.2,0.1, or 0.05, or a reduction in friction by any of the foregoing valuesor by at least 10, 20, 30, 40, 50, 60, 70, 80, or 90%. In otherembodiments, the ionic liquid or lubricating composition may or may nothave an appreciable effect on friction, but may reduce the wear rate,e.g., by at least or greater than 10, 20, 30, 40, or 50%. In yet otherembodiments, the ionic liquid or lubricating composition may or may notalso improve the corrosion resistance of the treated substrate. Theimproved corrosion resistance may be evidenced by a resistance tocorrosion in air or after treatment in a liquid corrosion test, such astreatment in a salt solution of at least 0.1 M, 0.2 M, 0.5 M, 1.0 M, 1.5M, or 2.0 M concentration for at least 0.5, 1, 2, 3, 4, 5, 6, 12, 18,24, 36, or 48 hours. In still other embodiments, the ionic liquidsdescribed herein may provide a multiplicity of functions, which can betwo or more of, for example, anti-wear, extreme pressure, frictionmodifier, anti-oxidant, detergent, and anti-corrosion functions.

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

EXAMPLES Example 1 Synthesis of the ionic liquidtrihexyltetradecylphosphonium 2-ethylhexanoate ([P₆₆₆₁₄][C₇H₁₅COO]b)

[P₆₆₆₁₄][C₇H₁₅COO]b was synthesized as follows. A 5 L three neckedjacketed flask fitted with an addition funnel, condenser, thermal welland nitrogen inlet was charged with [(hexyl)₃P(C₁₄H₂₉)]Cl (1079 g, 2.077mol) and 2-ethylhexanoic acid (329 g, 2.288 mol) and 390 g of toluene,and the mixture was heated to an internal temperature of ca. 50° C. Asolution of NaOH (116 g, 2.900 mol) in 200 g of water was added whilemaintaining the internal temperature at ca. 50° C. (addition over 1 h).The solution was stirred one hour further and was allowed to phaseseparate overnight. The aqueous phase was removed and the organic layerwas washed with 5×˜500 g water (to pH=7). Vacuum stripping of theorganic material to a maximum temperature of 160° C. and vacuum of 0.32mbarr afforded 1.141 kg of [P₆₆₆₁₄][C₇H₁₅COO]b (88% yield, 99% purity).¹H NMR (toluene-D₈, ppm): 0.50-0.75 (m, 1811, CH₃); 0.91-1.69 (m, 56H,CH₂); 2.01 (m, 1H, CH); 2.41 (m, 8H, PCH₂). ³¹P NMR (toluene-D₈, ppm):33.8 (s).

Example 2 Comparison of ionic liquid trihexyltetradecylphosphonium2-ethylhexanoate ([P₆₆₆₁₄][C₇H₁₅COO]b) with its straight-chain version,trihexyltetradecylphosphonium octanoate ([P₆₆₆₁₄][C₇H₁₅COO]s)

Density, Viscosity, Oil-Solubility, and Thermal Stability:

The oil-solubility in PAO 4 cSt base oil, decomposition temperature,density, and kinematic viscosity of [P₆₆₆₁₄][C₇H₁₅COO]b) (see FIG. 1A)are compared with those of a commercial amine-phosphate anti-wearadditive, [P₆₆₆₁₄][C₇H₁₅COO]s, where s denotes a straight chain (seeFIG. 1B), and three other phosphonium-carboxylate ionic liquids (ILs)with the same cation but different alkyls on the anion (see FIG. 2).

The solubility of the ILs in the PAO base oil was determined usingdirect observation after the centrifugation of the blends at 13,000 rpmfor 5 min. Among the three straight-chain-alkyl phosphonium-carboxylateILs, the IL with the long C17-alkyl ([P₆₆₆₁₄][C₁₇H₃₅COO]s) possesseshigh oil-solubility (>10%) however the other two ILs with C9- andC7-alkyls ([P₆₆₆₁₄][C₉H₁₉COO]s and [P₆₆₆₁₄][C₇H₁₅COO]s) are not solublein PAO (<1%). In contrast, both the two ILs with branched-chain alkyls([P₆₆₆₁₄][C₉H₁₉COO]b and [P₆₆₆₁₄][C₇H₁₅COO]b) are soluble in PAO tovarious extents, 2-5% and >10%, respectively. Evidently, the branchedalkyl structure is the game changer. Further, the longer branch (ethyl)of [P₆₆₆₁₄][C₇H₁₅COO]b compared to the methyl branch on[P₆₆₆₁₄][C₉H₁₉COO]b is believed responsible to the higher oil-solubilityof [P₆₆₁₄][C₇H₁₅COO]b (though its alkyl has a lower number of carbonsoverall C7 vs. C9). See Table 1.

TABLE 1 Oil-solubility, decomposition temperature, density, andkinematic viscosity Solubility Hildebrand Decomp. Density Kinematic inPAO solubility temperature @ 20° C. Viscosity (mPa · s) Lubricant (wt %)parameter (° C.) (g/mL) @ 40° C. PAO 4 cSt base oil — — 250 0.80 17.6Commercial amine- >10 — 310 0.92 1107 phosphate AW[P₆₆₆₁₄][C₇H₁₅COO]b >10 24.9 308 0.88 184.8 [P₆₆₆₁₄][C₇H₁₅COO]s  <1 26.0261 0.93 122.2 [P₆₆₆₁₄][C₉H₁₉COO]b >2, <5 24.8 258 0.91 127.1[P₆₆₆₁₄][C₉H₁₉COO]s  <1 25.5 NM 0.89 solid [P₆₆₆₁₄][C₁₇H₃₅COO]s >10 23.5308 0.88 174.5

Molecular dynamics simulation (Materials Studio 6.0) was used to computethe Hildebrand solubility parameters (δs) for the selected ILs and thePAO oil. The δ values of the five ILs in Table 1 are in the range of23.5-26.0. The PAO oil molecules were represented using two alkanes,decane and icosane, whose δs are 13.29 and 13.68, respectively.

Thermogravimetric analysis (TGA) was performed on a thermogravimetricanalyzer (TGA-2950, TA instruments) at a 10° C./min heating rate in air.All ILs possess higher thermal stability than the base oil. Thekinematic viscosities were measured on a Petrolab Minivis II viscometer.

Example 3 Corrosive Testing

Initial test results suggest that none of the phosphonium-carboxylateILs listed in Table 1 is corrosive to cast iron. A droplet of each ILwas placed on the surface of a piece of grey cast iron in the ambientenvironment for 7 days and no rusting was observed on any samplesurface.

Example 4 Superior Anti-Wear and Friction Reduction

The branched C7 phosphonium-carboxylate IL ([P₆₆₆₁₄][C₇H₁₅COO]b) wasmixed into the PAO base oil at 1.65 wt % and the wear protectionperformance was evaluated using a ball-on-flat tribological bench test.Results were benchmarked against the commercial amine-phosphate AWadditive at a treat rate of 1.67 wt % in the PAO base oil. Both fluidshad similar phosphorus content of ˜800 ppm (meeting the ILSAC GF-5specifications).

The tribo-tests were carried out on a reciprocating tribometer (PlintTE77, Phoenix Tribology Ltd.) by using a 10 mm AISI 52100 steel ballsliding against a CL35 cast iron flat. The flats were polished by usingP1200 SiC abrasive paper in distilled water with random motion. Testswere performed under a 100 N normal load and at a temperature of 100° C.The oscillation frequency used was 10 Hz with a stroke of 10 mm and thesliding distance was 1000 m. 2-3 repeat tests were carried out for eachlubricant.

The total wear rates (ball and plate) of the three fluids, neat PAO baseoil, PAO treated with the commercial amine-phosphate (PAO+1.67% AP), andPAO treated with [P₆₆₆₁₄][C₇H₁₅COO]b (PAO+1.65% IL) are compared in FIG.3. The branched C7 phosphonium-carboxylate IL significantly reduced thewear rate when added into PAO and even outperformed the commercialamine-phosphate AW additive by >56% wear reduction.

Example 5 Anti-Wear Tribofilm of [P₆₆₆₁₄][C₇H₁₅COO]b

A tribofilm formed on the worn surface lubricated byPAO+1.65%[P₆₆₆₁₄][C₇H₁₅COO]b, as shown in FIGS. 4 and 5. Relatively lowphosphorus concentration was detected in the tribofilm from eithercross-section TEM-EDS or top surface XPS chemical analyses, and resultssuggest that the tribofilm is primarily composed of iron oxides and ironcarboxylate complexes.

The cross-section TEM images (FIG. 4) suggest that the tribofilm is230-370 nm in thickness and contains a large amount of nano-particlesembedded in an amorphous matrix. The EDS cross-sectional elemental mapsshow high concentrations of oxygen and iron and just trace marks ofphosphorus.

The XPS spectra of Fe2p, O1s, C1s, and P2p on the surface lubricated byPAO-[P₆₆₆₁₄][C₇H₁₅COO]b are shown in FIGS. 5A-D. Two iron oxide peakswere identified: Fe (II) at 709.0 eV and Fe (III) at 710.9 eV. Asatellite peak from Fe (II) was found at 715.5 eV. The oxygen O1s peakwas resolved into two separate peaks: iron oxides and others possiblyincluding P═O, C—O, and C═O bonds. Three types of carbon groups wereidentified: carbon group at 284.9 eV, alcohol group at 286.4 eV, andcarboxylate group at 289.4 eV. It is believed that iron carboxylatecomplexes form when an iron surface interacting with carboxylates. Thesignal of P2p exhibited two peaks: 2p_(3/2) and 2p_(1/2). Decompositionof a phosphonium cation usually generates phosphine oxides, which mightfurther combined with Fe ions. Figure Se shows the XPS depth-compositionprofile of the tribofilm enabled by ion-sputtering, and results suggesta phosphorus content of 2-3 at %.

What is claimed is:
 1. An ionic liquid composition having the followinggeneric structural formula:

wherein R¹, R², R³, and R⁴ are each independently a hydrocarbon groupcontaining at least 4 carbon atoms, and R⁵ is a branched hydrocarbongroup containing 4 to 8 carbon atoms atoms.
 2. The ionic liquid of claim1, wherein the ionic liquid is soluble (>0.1% wt.) or fully miscible(>10% wt.) in a non-polar hydrocarbon lubricating oil.
 3. The ionicliquid of claim 1, wherein R¹, R², R³, and/or R⁴ is a hexyl group. 4.The ionic liquid of claim 1, wherein R¹, R², R³, and/or R⁴ is an octylgroup.
 5. The ionic liquid of claim 1, wherein the quaternaryphosphonium is trihexyltetradecylphosphonium [P₆₆₆₁₄].
 6. The ionicliquid of claim 1, wherein the quaternary phosphonium istetraoctylphosphonium [P₈₈₈₈].
 7. The ionic liquid of claim 1, whereinthe quaternary phosphonium is tributyltetradecylphosphonium P[₄₄₄₁₄]. 8.The ionic liquid of claim 1, wherein the quaternary phosphonium istributyltetraoctylphosphonium [P₄₄₄₈].
 9. The ionic liquid compositionof claim 1, wherein R⁵ is a branched heptyl group.
 10. The ionic liquidof claim 1, wherein the carboxylate is a 2-ethylhexanoate (C₇H₁₅COO).11. A lubricant composition comprising: (i) an ionic liquid having thefollowing generic structural formula:

wherein R¹, R², R³, and R⁴ are each independently a hydrocarbon groupcontaining at least 4 carbon atoms, and R⁵ is a branched hydrocarbongroup containing 4 to 8 carbon atoms; and (ii) a base oil; wherein saidionic liquid is dissolved in said base oil.
 12. The lubricantcomposition of claim 11, wherein said base oil is a mechanicallubricating oil.
 13. The lubricant composition of claim 11, wherein saidionic liquid is included in said base oil in an amount of at least 0.1wt %.
 14. The lubricant composition of claim 11, wherein said ionicliquid is included in said base oil in an amount of at least 1 wt %. 15.The lubricant composition of claim 11, wherein said ionic liquid isincluded in said base oil in an amount of at least 10 wt %.
 16. Thelubricant composition of claim 11, wherein said ionic liquid is includedin said base oil in an amount of at least 50 wt %.
 17. The lubricantcomposition of claim 11, wherein R¹, R², R³, and/or R⁴ is a hexyl group.18. The lubricant composition of claim 11 wherein R¹, R², R³, and/or R⁴is an octyl group.
 19. The lubricant composition of claim 11, whereinthe quaternary phosphonium is trihexyltetradecylphosphonium [P₆₆₆₁₄].20. The lubricant composition of claim 11, wherein the quaternaryphosphonium is tetraoctylphosphonium [P₈₈₈₈].
 21. The lubricantcomposition of claim 11, wherein the quaternary phosphonium istributyltetradecylphosphonium [P₄₄₄₁₄].
 22. The lubricant composition ofclaim 11, wherein the quaternary phosphonium istributyltetraoctylphosphonium [P₄₄₄₈].
 23. The lubricant composition ofclaim 11, wherein R⁵ is a branched heptyl group.
 24. The ionic liquid ofclaim 11, wherein the carboxylate is a 2-ethylhexanoate (C₇H₁₅COO).